1. Field of the Invention
The present invention relates to an image pickup apparatus applied to a video camera capable of image pickup at a variety of frame rates and to an image pickup method applied to such image pickup apparatus, and in particular to a technology suitable for an image pickup apparatus equipped with an image sensor that picks up an image in line sequence.
2. Description of the Related Art
In the past, a variety of video cameras that are capable of image pickup at different frame rates have been developed. There are a number of standards for the frame rate of an image signal, such as sixty frames per second, fifty frames per second, and twenty-four frames per second. Image signals at sixty frames per second are common in Japan and USA, while image signals at fifty frames per second are common in Europe. The frame rate at twenty-four frames per second is suitable for an image signal used in movies.
When image pickup is carried out corresponding to such a variety of image signal types using a single video camera, there are no problems so long as the base clock frequency of a synchronization signal used for image pickup (pixel clock frequency) can be changed in accordance with the different frame rates. However, this is not ideal since complex control is required to change the clock frequency in units of single pixels being picked up.
For this reason, it is customary to use the same base clock frequency for single pixel units when picking up images at different frame rates with the image sensor of a video camera, and to then process the picked-up image signal to convert the signal to a signal with a stipulated frame rate.
FIGS. 1A to 1D are diagrams showing an example of an existing frame rate converting process for the case where the base clock (pixel clock) frequency is fixed. The processing shown in FIGS. 1A to 1D is standardized according to SMPTE 274M or the like.
FIG. 1A is an image of one frame in an image pickup signal. A valid image pickup region d1 with predetermined numbers of pixels in the horizontal direction and the vertical direction is set in the image pickup signal of a frame. The numbers of pixels in the horizontal direction and the vertical direction in the valid image pickup region d1 are the same in each frame rate. As one example, the valid image pickup region d1 is assumed to include 1920 pixels in the horizontal direction and 1080 lines in the vertical direction.
When images are picked up at 60 frames per second, a slightly wider image pickup region d2 than the valid image pickup region d1 is set as shown in FIG. 1A. The image pickup region d2 is assumed to include 2200 pixels in the horizontal direction and 1125 lines in the vertical direction.
FIG. 1B shows a vertical synchronization signal V, a horizontal synchronization signal H, and an image signal of the valid region, when obtaining an image signal at 60 frames per second. For an image signal at 60 frames per second, an image pickup region d2 (that is, 2200 pixels in the horizontal direction and 1125 lines in the vertical direction) is repeatedly picked up in frame cycles. The image signal of the valid region is a signal of 1080 lines taken from a predetermined line (“n line”) out of the 1125 horizontal lines. A signal for one horizontal line is shown by the broken line Ha in FIG. 1A, for example.
When images are picked up at 50 frames per second, as shown in FIG. 1A, an image pickup signal of a region (d2+d3) produced by adding an image pickup region d3 on the right of the image pickup region d2 to the region d2 is outputted from a read unit of the image sensor. The range set as the valid image pickup region d1 is the same as for image signals at 60 frames per second.
FIG. 1C shows the vertical synchronization signal V, the horizontal synchronization signal H, and the image pickup signal of the valid region, when obtaining an image signal at 50 frames per second. When obtaining an image signal at 50 frames per second, an image of an image pickup region d2+d3 (that is, 2640 pixels in the horizontal direction by 1125 lines in the vertical direction) is repeatedly picked up in frame cycles. Here, in the same way as when images are picked up at 60 frames per second, the image pickup signal of the valid region is a signal of 1080 lines taken from a predetermined line (“n line”) out of the 1125 horizontal lines, but since the horizontal synchronization cycle is longer than when images are picked up at 60 frames per second, the part aside from the valid region in each horizontal line is longer. Here, the signal of one horizontal line is shown by the broken line Hb in FIG. 1A, for example.
When images are picked up at 24 frames per second, as shown in FIG. 1A, an image pickup signal of a region (d2+d3+d4) produced by further adding an image pickup region d4 on the right of the image pickup region d3 to the region (d2+d3) is outputted from a read unit of the image sensor. The range set as the valid image pickup region d1 is the same as for image signals at 60 frames per second.
FIG. 1D shows the vertical synchronization signal V, the horizontal synchronization signal H, and the image pickup signal of the valid region, when obtaining an image signal at 24 frames per second. When obtaining an image signal at 24 frames per second, an image of an image pickup region d2+d3+d4 (that is, 2750 pixels in the horizontal direction by 1125 lines in the vertical direction) is repeatedly picked up in frame cycles. Here, in the same way as when images are picked up at 60 frames per second or 50 frames per second, the image pickup signal of the valid region is a signal of 1080 lines taken from a predetermined line (“n line”) out of the 1125 horizontal lines, but since the horizontal synchronization cycle is even longer than when images are picked up at 50 frames per second, the part aside from the valid region in each horizontal line is extremely long. Here, the signal of one horizontal line is shown by the broken line Hc in FIG. 1A, for example.
By carrying out such image pickup processing, as the frame rate falls, the part that is not the valid image pickup region in each horizontal line (that is, the invalid region) increases, which makes it possible to change the frame rate even if the base clock frequency is fixed. That is, when obtaining an image signal at 50 frames per second or at 24 frames per second, the invalid regions (i.e., the regions d3, d4) are deleted from the image pickup signal read from the image sensor, and a correction process is carried out to extend the time axis of each horizontal line, and thereby an image signal with the stipulated frame rate is obtained.
In the processing shown in FIGS. 1A to 1D, it is possible to pick up images at a variety of frame rates by changing the horizontal synchronization cycle without changing the base clock frequency for driving the image sensor or read unit of the image sensor. By handling changes to the frame rate by changing the horizontal synchronization cycle without changing the base clock frequency in this way, control that changes the base clock frequency becomes unnecessary and it is possible to prevent flickering on the display.
PCT Publication WO2003-63471 discloses an existing image pickup apparatus that carries out control to change the frame rate. The publication states that by carrying out control that does not change the base clock frequency even when the frame rate changes, it is possible to prevent flickering on the display.